Recording carriers or recording media, typically having a paper base with a metallic coating thereover, have been described in the literature, see, for example U.S. Pat. No. 3,995,083, which discloses such a carrier using an intermediate layer which includes printing ink in order to increase the contrast of recorded information when the metal coating thereon, typically aluminum, is burned off. Using printing ink which can be colored, preferably black by using carbon black, provides good contrast and also has a certain surface roughness which reduces the sheen of the metallic aluminum coating on the paper.
It has also been proposed--see U.S. Pat. No. 3,831,179--to place an intermediate layer of a laquer, typically cellulose lacquer, as a contrast layer between a paper substrate or paper web and a metallic top coating, typically aluminum. The lacquer layer has particularly good blocking action with respect to migration of vapors between the substrate and the metallic coating and thus is eminently suitable to prevent corrosion of the aluminum coating, thereby increasing the shelf life of such recording media. The lacquer coating has some disadvantages, however, in that contrast of recorded information and surface sheen of the overall paper are not always satisfactory.
It has been proposed to increase the contrast, particularly when using printing ink as the intermediate layer, by adding carbon black as a contrast and matte surface producing agents to the ink. The quantity of carbon black which can be added to the ink is not unlimited, however, since carbon black is an electrical conductor and, if the percentage thereof becomes too great, for example in excess of about 15%, significant current can flow through the carbon black additives, resulting in undesired heating of the intermediate layer. Both lacquer and printing ink frequently are made on the basis of nitrocellulose, also known as gun cotton. During recording, an exothermal reaction may occur in which the intermediate layer is involved, destroying the recording carrier substrate in this region.
A frequently used additive to cause a matte surface is based on silicon oxide--commonly a suitable SiO.sub.x compound--which, however, causes difficulty in recording. During the recording process, the top layer is burned off; SiO.sub.x compounds will be affected similar to a sintering process, causing deposits on the electrodes which substantially decreases their useful life. The writing current may even be interrupted, causing an interruption in recording. Removal of this deposit by increasing the writing pressure is undesirable since it increases wear on the recording medium and further on the electrodes themselves.